A method for preparing a precursor for electrolytes and a method for producing electrolytes for an all-iron flow battery

Abstract

The claimed group of inventions relates to the energy industry, particularly, to electrochemical technologies for energy accumulation and storage, and it relates to a method for preparing a precursor for electrolytes of an all-iron flow battery and to a method for producing electrolytes based on this precursor. According to the first invention, the method for preparing the precursor comprises steps of mixing an iron chloride crystal hydrate and distilled water, while simultaneously passing an inert gas through a solution during its formation, separating the solution into two parts and reducing Fe3+ to Fe2+ in one of the parts, and oxidizing, in another part, Fe2+ ions to form Fe3+ ions in an electrochemical cell during a charging process. At the same time, an excess of a reducing agent may be added to the solution prior to the step of separating. The claimed method allows to purify the reduced part of the solution from Fe impurities and to stabilize a content of Fe2+ ions therein. According to the second invention, the electrolytes are produced from the reduced part of the precursor solution by adding electrically conductive additives and successively adjusting an acidity to a specific limit. The electrolytes are produced under vigorous stirring and heating, while the negative electrolyte is produced in an inert gas or hydrogen atmosphere. Said solution allows to provide a stable composition of the electrolytes and to ensure a high round-trip efficiency of the all-iron battery during multiple charge- discharge cycles at a high current density and achievement of a high total capacity.

Description

[0001] A METHOD FOR PREPARING A PRECURSOR FOR ELECTROLYTES AND A METHOD FOR PRODUCING ELECTROLYTES FOR AN ALL-IRON FLOW BATTERY

[0002] FIELD OF THE INVENTION

[0003] The claimed group of inventions relates to the energy industry, particularly, to electrochemical technologies for energy accumulation and storage, and it relates to a method for preparing a precursor and a method for producing iron-based electrolytes to be used in a system of an all-iron flow battery. The claimed invention is a partial continuation of the applications a202300982 titled “A method for regenerating electrolytes of an all-iron flow battery” and a202300983 titled “A system for regeneration of electrolytes of an all-iron flow battery”, as well as of the application a202302087 titled “A cell of an all-iron flow battery” dated May 2, 2024, and their content is included into the claimed invention by reference.

[0004] PRIOR ART

[0005] Redox flow batteries are electrochemical accumulators which are characterized by use of aqueous solutions of electrolytes, store electrical energy in a chemical form and convert the stored chemical energy into an electric form by means of redox counter reactions which are induced by use of electric current. In terms of structure, the flow batteries consist of an electrochemical cell that is separated into chambers by a membrane barrier, while positive and negative electrodes are arranged in the chambers. The membrane barrier separates positive and negative sides, thereby ensuring a transfer of balancing ions between the electrolytes when an electrical circuit is present. The chambers are filled with positive and negative electrolytes, and each of the chambers is connected to a container for storing the corresponding electrolyte, while enabling a closed circulation of the electrolytes between the containers and the cell chambers. When a charging current is supplied, the electrolyte ions are brought into contact with electrodes and waste electrons on the positive electrode and receive electrons on the negative electrode, while redox counter reactions take place during a discharge upon formation of the electrical circuit and, thus, occurrence of the discharge current on the electrodes and, therefore, in the electrolytes. The formed closed electrochemical system may accumulate and convert the electrical energy into chemical energy and vice versa on a multiple basis, thereby providing its reliability and scaling possibilities. The flow batteries are used in grid electrical energy storage systems which may be used to decrease peak loads, to balance loads, to provide standby power supply, etc. Due to development of the flow batteries field, it is allowed to advance and scale the storage and use of energy from sources of non-uniform generation such as solar, wind, or geothermal energy.

[0006] Currently, a promising option is to use all-iron flow batteries due to the abundance of raw materials, their affordability, as well as environmentally friendly production and usage. Owing to the electron configuration of iron, it may be deposited into a uniform structure during its galvanic deposition onto a substrate of the negative electrode, thereby providing a stable morphology of the electrode. Therefore, the all-iron flow batteries decrease the use of toxic raw materials as compared to other redox flow batteries, thereby decreasing an environmental hazard correspondingly. Electrochemical reactions involving iron ions which are dissolved in the corresponding electrolytes take place on the positive and negative electrodes of the all-iron flow battery. Aqueous solutions of iron chlorides or sulfates comprising non-active supplementary electric conductivity additives and having an adjustable acidity may be used as active redox solutions for preparing the electrolytes of the all-iron battery, thereby ensuring that the positive electrolyte and the negative electrolyte for the positive electrode and negative electrode, respectively, comprise iron in various valent states which ensures their electrochemical interaction. Precursors for preparing the electrolytes for the alliron batteries are FeCl2, FeCl3, FeSO4, Fe2(SO4)3solutions, while the supplementary additives are NaCl, KC1, NH4C1, or Na2SO4. The positive electrode uses a Fe / Fe redox pair of the positive electrolyte, while the negative electrode uses a Fe2+ / Fe° redox pair of the negative electrolyte. The amount of energy that is stored by the redox battery depends on the amount of an electrically active material that is available in the electrolytes for discharging depending on a total volume of the electrolytes and solubility of the electrically active materials.

[0007] However, a crossover of the active substances may occur in the electrolytes, and it is caused by two side reactions on the negative side of the battery that result in a reduction of hydrogen protons H+to form gaseous hydrogen H2and a reduction reaction to reduce Fe3+to Fe2+ions, thereby increasing pH of the negative side electrolyte and deposition of the iron oxides on the membrane which decreases a total efficiency of the battery.

[0008] Therefore, a composition and condition of the electrolytes, especially, a stability of the content of Fe ions which are comprised in both redox pairs in parent solutions, play an important role in ensuring the round-trip efficiency of the all-iron batteries.

[0009] Methods for increasing the stability of electrolytes are disclosed, e.g., in a patent US10586996B2 dated March 10, 2020, where the stability of a negative electrode is increased by periodical addition of an acidic additive from an additional reservoir, or in a patent EP3583648B1 dated June 12, 2024, where, besides the addition of the acidic additive, an inert gas is supplied, in a dosed fashion, to a free space of electrolyte storage tanks for displacing oxygen and decreasing oxidation of the electrolytes. These methods require introduction of additional control elements into the all-iron battery system for controlling the condition of the electrolytes and individual reservoirs for storage and supply of the required additives in a defined period, thereby making the system more expensive and complicating the technology due to mandatory pressure control in the system during the inert gas supply.

[0010] Also, a method for increasing a stability of electrolytes as disclosed in an application US2023086739A1 dated March 23, 2023, is known, where the stability of negative and positive electrolytes is adjusted by preparing them on a base of two precursors having different metal concentrations. For example, in some embodiments, the metal precursor in the negative electrolyte comprises FeCl2at a concentration of 1.0-4.5 M and FeCl2at a concentration of 0.5-4.0 M in the positive electrolyte. The negative electrolyte is prepared by using a deaerated solution of FeCl2-4H2O having said concentration of FeCl2followed by addition of NH4C1 and subsequent addition of glycine at a continuous purge with N2, while pH is adjusted approximately to 1.5 by means of HC1. The volume is adjusted to reach desired electrolyte concentrations. The positive electrolyte is prepared by using a deaerated solution of FeCl2'4H2O having said concentration of FeCl2followed by addition of NH4C1 and subsequent addition of glycine. All the steps are carried out at the continuous purge with N2. The pH was adjusted approximately to 1.5 by means of HC1. The volume was adjusted to reach desired concentrations.

[0011] Tests of the battery on said electrolytes have demonstrated a decreased cross-over of active particles, a decreased hydrogen release and an improved coulombic, voltaic, and round-trip efficiency.

[0012] Said solution still has a problem of the content of the oxidized form of Fe in the parent solution of FeCl2that cannot be fully resolved by adding an organic additive, i.e., glycine. Besides, glycine itself as a complexing substance increases the overload of the iron reduction process, thereby decreasing the voltaic efficiency of the all-iron flow battery.

[0013] A patent US11837767B2 dated December 5, 2023, may be chosen as the closest prior art of the claimed solution, and this patent discloses a method for preparing a precursor, where solutions that are obtained by reacting iron-containing salts FeCl2, FeSO4, FeO, Fe with distilled water are used as the precursor, and further preparing, based on them, negative and positive electrolytes for the all-iron battery. According to this solution, no additional influence on the precursor is conducted, while the increase of the voltaic, current, and total efficiency is influenced by adding an amino acid glycine as a stabilizing additive to the positive electrolyte, as provided in the previous solution. The best results which are illustrated in this solution were obtained upon investigation of 40 (forty) 4-hour • • * 2 operation cycles of the all-iron battery at a current density of 30 mA / cm , and they were as follows: coulombic efficiency (CE) = 92.4% voltaic efficiency (VE) = 81.1%, round-trip efficiency (RTE) = 74.9%, which provided an average 14% increase of the operation parameters as compared to the all-iron battery on the electrolytes of a similar composition without any stabilizing additive.

[0014] However, use of the low current density (30 mA / cm ) and short-term cycles (4 hours), while achieving a round-trip efficiency of 70-75%, indicates that the stability of the electrolytes is insufficient which may be caused by an increase of overloads in the charging / discharging processes due to use of the organic additive in their composition which, in turn, results in a decrease of the total energy efficiency of the battery operation. Possibly, it is caused by the fact that glycine makes Fe3+compounds soluble in water, thereby decreasing the coulombic efficiency of the all-iron flow battery, since these compounds become electrochemically active during the battery charging process. Anyway, the total capacity that equals 120 mA'h / cm2in this solution is not sufficient to resolve the problem of electrical energy accumulation, thus, there is a need to improve the condition of the electrolytes to ensure a long-term use of the electrolyte during a large number of charge-discharge cycles and to increase the total capacity.

[0015] SUMMARY OF THE INVENTION An underlying objective of the claimed group of inventions is to prepare iron-based electrolytes which represent a mixture of a precursor and active electrically conductive materials and have improved composition stability both at the beginning and during long-term operation of the all-iron battery by decreasing a content of Fe3+ions in a target solution of the precursor and substantially obtaining the Fe precursor which will allow creating, on its basis, electrolytes for conduction of redox reactions without formation of any insoluble Fe compounds in all operation steps of the all-iron battery.

[0016] A technical effect that is achieved by resolving the objective is preservation of the stable composition of the electrolytes during multiple charge-discharge cycles of the all-iron battery at a high current density and, thus, achievement of a high total capacity and duration of the battery operation period without interruption for maintenance work to restore the electrolytes and the membrane.

[0017] The objective is achieved by implementing two aspects of the invention, where the first aspect relates to a method for preliminary preparation for production of electrolytes, namely, to a method for preparing a precursor having a content of Fe2+ions by purifying it from impurities of Fe3+ions. According to the claimed invention, the method comprises steps of mixing an iron chloride crystal hydrate and distilled water to obtain FeCl2solution, while simultaneously sparging an inert gas through the solution, separating the solution into two parts, and further at least one step of reducing, in a first part of the solution, Fe ions to Fe ions, while simultaneously oxidizing, in a second part of the solution, Fe ions to form Fe3+ions by passing the parts of the solution through an electrochemical cell of an all-iron battery during a charging process, while applying a current of 0.1-10 A per 1 cm2of the electrochemical cell until values of a constant voltage of 1-1.4 V and a current of 0.5-5 A are achieved after an upper permissible voltage threshold is reached, and further transferring the reduced part of the solution to a container filled with inert gas or hydrogen. Therewith, preferably, the first part of the solution is 55-75% of a total volume of the FeCl2solution. • 3+ •

[0018] Since Fe ions are present in raw materials and their concentration increases during preparation of the FeCl2solution, the sparging with gaseous argon displaces water- and air-soluble oxygen from the container, where mixing of the solution takes place, and decreases an oxidation level to Fe already in a primary stage of the precursor preparation. The next step of processing the part of the solution during charging facilitates its complete purification from Fe3+ions which reduce to Fe ions, while its storage in the inert gas atmosphere, e.g., argon or nitrogen, completely protects the precursor solution against oxidation for a certain period of time. Therefore, said method for preliminary processing of the precursor allows to prepare the solution having a stable content of Fe ions that is ready for further use, i.e., for producing electrolytes with a high efficiency.

[0019] Therewith, another part of the FeC , solution, where the process of oxidation of Fe ions takes place, subsequently may be either used for repeated reduction and production of the precursor or added during preparation of the next portion of the precursor.

[0020] According to one of possible exemplary embodiments of the invention, prior to the step of separating into two parts, an excess of a reducing agent is added to the solution at a ratio 1 : n, where n is a mass of the reducing agent and n = 0.05- 0.005 of a mass of FeC that is dissolved in the distilled water, the obtained mixture is maintained for 1-24 hours at a temperature of 20-90°C in the inert gas or hydrogen atmosphere, while stirring continuously, and prior to the step of separating, the solution is filtered. The addition of the reducing agent ensures an additional decrease of an excessive concentration of Fe impurities in the FeCl2solution obtained from the crystal hydrate and the distilled water after influence by the inert sparging, thereby decreasing a need in the repeated reduction of the solution in the electrochemical cell. Therewith, an iron powder and / or hydrazine are / is used as the reducing agent. Selection of the reducing agent is explained by the fact that the iron powder and hydrazine do not remain in the reduced solution, since Fe° residues are filtered out, while hydrazine is decomposed into N2and H2O.

[0021] Another aspect of the claimed invention relates to a method for preparing solutions of electrolytes for an all-iron flow battery using the precursor obtained according to the first aspect of the claimed invention. According to the claimed invention, the negative electrolyte is produced from the reduced part of the precursor solution by adding an electrically conductive additive NH4C1 at a concentration of 0.5-4.0 M and a buffer additive H3BO3 at a concentration of 0.05- 1 M, while adjusting pH to values in a range of 1.0-5.0, and possibly adding a distilled water and then adjusting the pH to values in a range of 2.0-5.0 and filtering, and all the steps are performed in an inert gas or hydrogen atmosphere, while stirring vigorously and heating to a temperature of 30-80°C. The positive electrolyte is prepared by adding, to the reduced part of the precursor solution, FeCl3-6H2O and the electrically conductive additive NH4C1 of 0.5-4.0 M, while adjusting pH to values in a range of 0.5- 1.0, and possibly adding a distilled water and then adjusting pH to values in a range of 0.0-1.5, filtering, and all the steps are performed under vigorous stirring and heating to a temperature of 30-80°C. Therewith, the parts of the reduced precursor solution are separated in such a way that the volume of the obtained negative electrolyte is greater than the volume of the positive electrolyte that is caused by different throughput capacity of the electrochemical cell and, thus, a need in a certain amount of electrolytes.

[0022] Said sequence of actions avoids any local increase of pH and formation of insoluble compounds in both electrolytes, as well as avoids any loss of the Fe concentration in the negative electrolyte, thereby ensuring a stable balance of the electrolytes during their subsequent use which, in turn, decreases the influence on the ion-exchange membrane of the battery by decreasing its ohmic resistance and, thus, prolongs the capacity of the all-iron battery.

[0023] DESCRIPTION OF THE DRAWINGS In order to provide a more complete understanding of the invention and advantages thereof, the following description provides an explanation of possible exemplary embodiments of the invention with a reference to the appended figures, wherein identical designations denote identical parts:

[0024] Fig. 1 schematically illustrates an arrangement of the all-iron battery system for preliminary processing the precursor solution,

[0025] Fig. 2 schematically illustrates the all-iron battery system with a system for regeneration of electrolytes;

[0026] Fig. 3 illustrates a visual appearance of the precursor solution without preliminary processing (a) and after purification from Fe3+impurities (b)

[0027] Fig. 4 illustrates the influence of NH4SCN on the precursor solution without preliminary processing (a) and after purification (b),

[0028] Fig. 5 illustrates a visual appearance of the precursor solution-based electrolytes without preliminary processing (a) and after purification (b),

[0029] Fig. 6 illustrates a plot showing the round-trip efficiency and the voltage during operation of the flow battery without application of the preliminary processing,

[0030] Fig. 7 illustrates a plot showing the round-trip efficiency and the voltage during operation of the flow battery with application of the preliminary processing,

[0031] Fig. 8 illustrates a visual appearance of the membrane after 10 (ten) 6-hour operation cycles of the flow battery without application of the preliminary processing,

[0032] Fig. 9 illustrates a visual appearance of the membrane after 20 (twenty) 6- hour operation cycles of the flow battery with application of the preliminary processing.

[0033] Main designations:

[0034] 1. Electrochemical cell of the all-iron flow battery

[0035] 2. Negative side chamber 3. Positive side chamber

[0036] 4. Membrane of the cell of the all-iron flow battery

[0037] 5. Negative side tank

[0038] 6. Positive side tank

[0039] 7. Negative side circulation circuit

[0040] 8. Positive side circulation circuit

[0041] 9. Negative side pump

[0042] 10. Positive side pump

[0043] 11. Magnetic stirrer

[0044] 12. Iron-oxygen electrolyzer

[0045] 13. Iron-hydrogen battery

[0046] POSSIBILITY OF IMPLEMENTATION OF THE INVENTION

[0047] At the beginning of a method for preparing a precursor, it comprises a step of preparing a FeCl2solution at a concentration of 3.5 -4.0 M in a separate container equipped with a sparger by dissolving a crystal hydrate, e.g., FeCl2-4H2O, in a distilled water, while continuously passing argon during dissolving of said crystal hydrate. Thereafter, the obtained solution is separated into two parts which are then poured into tanks of an all-iron battery system that is used as an arrangement for performing the next step of preparation. The all-iron flow battery system for preliminary processing of the precursor solution is schematically illustrated in Fig. 1 and consists of main elements such as an electrochemical cell (1) that is separated into a positive chamber (2) and a negative chamber (3) having corresponding electrodes, and an ion-exchange membrane (4) that provides an electrical contact between the solutions in the cell upon application of a current and prevents them against mixing. The chambers (2, 3) are connected to corresponding tanks (5, 6), thereby forming separate closed circuits (7, 8) which are equipped with circulation pumps (9, 10) at an inlet of the chambers (2, 3). Therewith, the tank (6) is further equipped with a magnetic stirrer (11) that is used for stirring the solution being reduced in order to avoid its layering and allows to achieve the most optimal reduction time. A smaller part of the solution that constitutes 25-45% of the total volume of the FeCl2solution is placed into the positive side tank (5), while a greater part of the solution that constitutes 55-75% of the total volume of the FeC12 solution is placed into the negative side tank (6), and the pumps (9, 10) are started to pump the parts of the solutions along the circuits (7, 8), while passing them through the corresponding chambers of the electrochemical cell (1). A difference between the volumes is caused by a lower throughput capacity of the positive chamber (2) and, thus, by a greater throughput capacity of the negative chamber (3) of the electrochemical cell (1). At the same time, the current is passed through the electrodes of the cell, and an electrode of the negative chamber acts as an anode, while an electrode of the positive chamber acts as a cathode, and an electrochemical interaction process takes place, thereby ensuring a transfer of balancing ions between the solutions. Upon supply of the current, a direct oxidation reaction of Fe ions takes place in the positive side solution that results in a formation of Fe ions in the solution, while a counter reaction Fe2+±5 Fe3+ +e takes place in the negative side solution that results in achievement of an electrode potential +0.77 V that is a standard for this redox pair. This reaction is caused both by diffusion of Fe3+ions from the positive side towards the negative side through the electrochemical cell and by a possible presence of residual Fe3+ions in the output solution that is a side and generally undesired effect during operation of the all-iron battery, but is substantially crucial for obtaining the solution having a maximum concentration of Fe2+ions on the negative side being the solution that will be used as the precursor for preparing the electrolytes.

[0048] In order to reduce the solution, the current is passed between the negative and positive side electrodes of the electrochemical cell from a current source of 10- 200 A to the electrochemical cell having an area in a range of 20-1000 cm in an operation mode with application of a maximum possible current of 2-200 A. After a constant voltage of 1-1.4 V is achieved, the reduction process is continued at the constant voltage until the current of 0.5-5 A is achieved. The “2-200 V” operation

[0049] Q_l_ mode shall mean that during the reduction process, the concentration of Fe ions will be decreased, therefore, at the constant voltage of 1-1.4 V at the given voltage range of 10-200 A, the system may consume the current of 2-200 A, and this value is set depending on dimensions of the electrochemical cell. In order to intensify the reduction process, it is carried out under continuous stirring by means of the magnetic stirrer (11) that is provided in the negative side tank (6). A visual comparison of the precursor solution without application of the preliminary processing and the precursor that is obtained by the claimed method is illustrated in photos in Fig. 3, where a visual appearance of the precursor solution without preliminary processing is depicted on the left, and the one after purification from Q i

[0050] Fe impurities is depicted on the right.

[0051] Except for the visual comparison, a reaction inspection of a quality of the obtained precursor solution for a presence of possible residues of Fe ions and its oxo- and / or hydroxo-compounds was carried out. The inspection was carried out both immediately after preparation of the solution and 2-3 hours after preparation of the solution by titration thereof with ammonium rhodanide with the following reaction:

[0052] FeCl3+ NH4SCN -► Fe(SCN)3+ NH4C1.

[0053] To this end, 1.0-50.0 ml of the NH4SCN solution at a concentration of 1 M was titrated with the obtained Fe2+precursor solution with an increment of 0.05-0.1 ml. If the color of the NH4SCN solution during the titration process remained unchanged upon addition of 0.5-50.0 ml of the precursor solution, then this solution was considered as purified from Fe3+and its oxo- and / or hydroxo- compounds (e.g., Fe2O3, Fe(OH)3) and ready for further use. If the color was changed from colorless to red, the reduction was repeated. Therewith, it was not observed that the repeated reduction is required in any case of titration of the newly prepared solution and it was observed in two of ten cases of postponed titration which means that the used method for preparing the precursor is effective. Fig. 4 illustrates an interaction between NH4SCN and the FeCl2solution without any preliminary processing that is usually used for preparing electrolytes (a) and the Fe precursor after preparation by the claimed method (b).

[0054] In the course of studies, it has been also determined that it may be effective for achievement of the target precursor solution to combine the above-described electrochemical preparation step with a thermochemical step of preliminary processing. This step comprises adding a reducing agent to the FeCl2solution, thereby facilitating additional purification of the solution from the Fe impurities. During formation of the solution, an excess of the reducing agent is added at a ratio 1 : n, where n is a mass of the reducing agent and n = 0.05-0.005 of the mass of FeCl2, i.e., if 1 kg of FeCl2is taken for the preparation of the precursor solution, then it is required to provide 0.05-0.005 kg of the reducing agent for the total volume of the obtained solution, the obtained mixture is maintained for 1-24 hours at a temperature of 20-90°C in the inert gas, e.g., argon or nitrogen, or hydrogen atmosphere, while stirring continuously during 3-6 hours. Prior to separation, the solution is filtered through a paper or ashless filter. An iron powder (99.9% Fe°) or hydrazine (N2H4) may be used as the reducing agent. Selection of the reducing agent is explained by the fact that the iron powder and hydrazine do not remain in the reduced solution, Fe° residues are filtered out, and hydrazine is decomposed into N2and H2O. The quality of the reduction, i.e., of the purification from the Fe3+impurities, is inspected with ammonium rhodanide according to the abovedescribed method, and 90% of the inspection results do not demonstrate any presence of Fe3+ions. This step may be effectively used to decrease the number of cycles of the solution reduction in the electrochemical cell. Titration of the precursor solution with the additional processing step or with the repeated reduction step resulted in the color change only after 8-10 hours of its storage, thereby indicating the long-term stability of the precursor. In order to avoid quick oxidation and any future influence on the quality of the electrolytes, the obtained Fe2+precursor solution is stored in a tightly closed manner in the inert gas (argon or nitrogen) or hydrogen atmosphere, and it can be used, with consideration of the titration results, during 6-8 hours in average, while the reduction can be repeated after that.

[0055] The obtained results indicate that the claimed method for preparing the precursor as the target solution for preparing electrolytes is effective.

[0056] In order to prepare the negative electrolyte, according to the claimed invention, weighed samples of the electrically conductive additive NH4C1 and the buffer additive H3BO3 are dissolved in the prepared precursor solution, while ensuring that the obtained concentration of NH C1 is 0.5-4.0 M and of H3BO3 is 0.05-1 M. The solution is stirred at a rate of 180-400 rpm at a temperature of 30- 80°C in an inert medium, e.g., in a presence of argon or nitrogen, or in a hydrogen medium. After solids are dissolved, 5-10% of the NH3H2O solution is added to the formed solution and pH is adjusted to values in a range of 1.0-5.0, and these steps are also performed under vigorous stirring, heating, and in the inert gas or hydrogen atmosphere. The volume of the obtained electrolyte solution is measured, and, if necessary, distilled water is added, and pH is again adjusted to values in the range of 2.0-5.0 with 5-10% of the NH3H2O solution. The obtained electrolyte solution is filtered through a dual ashless “blue stripe” filter or a paper filter, while continuously passing the inert gas through the solution in a funnel. Said sequence of actions avoids any local increase of pH, formation of insoluble iron compounds and loss of the Fe2+concentration, while the presence of the gaseous medium allows to decrease the amount of the dissolved oxygen in the negative electrolyte and in the free space of the container without reacting with the electrolyte. Prior to mounting of the all-iron battery in the negative tank, the negative electrolyte solution is stored in a tightly closed manner and in the inert atmosphere. The entire preparation process is carried out using the container that is made of an acid- resistant material, glass or plastic, and that is equipped with a sparging arrangement, a pH electrode, a magnetic stirrer and a heating element. In order to prepare the positive electrolyte that consists of FeCl2at the concentration of 1.20-1.40 M, FeCh at the concentration of 0.10-0.30 M and the electrically conductive additive NH4C1 at the concentration of 0.5-4.0 M, weighed samples of FeC13-6H2O and NH4C1 are dissolved in the prepared precursor solution under vigorous stirring at the rate of 180-400 rpm at the temperature of 30-80°C, and after solids are dissolved, pH is adjusted to values in the range of 0.5- 1.0 by means of the 25-35% HC1 solution. Afterward, the volume of the obtained electrolyte is measured, the necessary volume of the distilled water is added, and pH is adjusted to values in the range of 0.5- 1.5 by means of the 25-35% HC1 solution under vigorous stirring and heating. The obtained electrolyte solution is filtered through the dual ashless “blue stripe” filter or the paper filter. Prior to mounting in the battery, the positive electrolyte solution is stored in a tightly closed manner and at a room temperature.

[0057] The entire preparation process is carried out using the container that is made of an acid-resistant material, glass or plastic, and that is equipped with a pH electrode, a magnetic stirrer and a heating element.

[0058] Even visual comparison between the obtained electrolyte solutions and the electrolytes obtained without any preliminary preparation of the precursor and in a traditional mode of addition of electrically conductive buffers indicate the difference between them as illustrated in Fig. 5 - without the preliminary processing (a), with the preliminary processing (b).

[0059] In order to compare the quality of the electrolytes obtained with and without application of the methods of the present invention, the operation of the all-iron battery equipped with the system for controlling a stability of electrolytes that is disclosed in the application a202300983 dated March 10, 2023 has been tested, the system further eliminates an electrolyte derangement that is caused by side reactions of redox pairs during operation of the battery due to provision of an ironoxygen electrolyzer (12) that stabilizes a nominal pH value of the negative electrolyte during operation and an iron-hydrogen battery (13) that operates using je

[0060] Fe ions of the positive electrolyte and hydrogen that releases as a by-product at the negative side electrode.

[0061] In order to compare the results of the operation of the all-iron battery, the electrolytes were prepared from the precursor solution with and without application of the preliminary processing.

[0062] The composition of the electrolytes: the negative electrolyte: 0.5-4.0 M of FeC ; 0.5-3.0 M of NH4C1; 0.05-1.0 M of H3BO3 - 80-200 ml the positive electrolyte: 0.25-2.0 M of FeCh; 0.10-0.30 M of FeCl3; 0.5-3.0 M of NH4CI - 160-200 ml

[0063] In order to test the round-trip efficiency and the cyclic stability of the system, cells having an area of 10-50 cm were used, and they were tested at a current density of 70-250-70 mA / cm2and during 6-hour cycles. With this initial data, voltage limits of 1.22-1.35 V (at the charge) and 1.10-0.50 V (at the discharge) are achieved. In particular, the battery operation was tested at the cell having an area of 20 cm at the current density of 70 mA / cm and at the temperature of 65-70°C.

[0064] In order to calculate the round-trip efficiency, values of coulombic and voltaic efficiencies are used.

[0065] Let’s designate the coulombic efficiency as CE (Coulombic efficiency):

[0066] CE=Q_(discharge) / Q_(charge) 400%

[0067] Qdischarge - discharge capacity, amount of electricity that is released at the discharge during a certain period of time (A-h);

[0068] Qcharge - charge capacity, amount of electricity that is consumed at the charge during a certain period of time (A h).

[0069] Let’s designate the voltaic efficiency as VE (Voltaic efficiency):

[0070] VE=U_(discharge) / U_( charge) 100%

[0071] Udischarge - average discharge voltage (V);

[0072] Ucharge - average charge voltage (V). The round-trip efficiency is designated as RTE (round-trip efficiency) and calculated as a product of the coulombic efficiency and the voltaic efficiency:

[0073] RTE-CE- VE— (Q_(discharge‘) U_(discharge)) / (Q_(charge’)

[0074] )T00 / o_(P_(discharge') ^_(dischargc)) / (P_(charge’) ^_(charge') )’100%

[0075] Pdischarge - discharge power (W);

[0076] Pcharge - charge power (W);

[0077] ^discharge - discharge time (h);

[0078] ^charge - charge time (h).

[0079] According to the results of 10 (ten) six-hour operation cycles of the battery, average values of the round-trip efficiency of the battery based on electrolytes without the preliminary processing were 67.33 % and 70.92 % with the preliminary processing, while the coulombic efficiency when using the electrolytes without the preliminary processing was 94.62 %, while it was 94.42% with the preliminary processing.

[0080] Plots of dependency between the round-trip efficiency and the voltage are illustrated in Fig. 6 and Fig. 7 of the drawings.

[0081] Fig. 6 illustrates a change on the voltage and the value of the round-trip efficiency of the flow battery with the electrolytes that were prepared from FeCl2without the preliminary processing. It may be seen that the decrease of the roundtrip efficiency has a downgrade trend, and the significant decrease below 70% is seen starting from the 6th cycle (marked by a dashed line) that is particularly associated with the increase of the charge voltage and with the decrease of the discharge voltage (marked by solid line).

[0082] Fig. 7 illustrates the change of the voltage and the value of the round-trip efficiency of the flow battery with the electrolytes which were prepared from FeCl2with application of the preliminary processing as disclosed in the first aspect of the claimed invention and prepared according to the second aspect of the claimed invention. In this case, the round-trip efficiency value is higher than 70% during all the mentioned cycles (marked by dashed line) and later, while the voltage change is uniform (marked by solid line).

[0083] Based on the mentioned information, it can be concluded that in case of a minor decrease at the end of the cycles of the coulombic efficiency, the round-trip efficiency increases, and this is caused by achievement of the voltage uniformity during multiple cycles of the charge / discharge process.

[0084] Therewith, the condition of the membrane after 10 (ten) operation cycles of the battery with electrolytes on the non-processed solution as illustrated in Fig. 8 indicates a high content of iron hydroxide therein, and it likely was the reason for the system degradation and efficiency decrease. While the membrane, according to the results of 20 (twenty) six-hour operation cycles of the battery with the electrolytes obtained according to the claimed invention from the preliminary processed precursor solution as illustrated in Fig. 9 indicates an insignificant amount of the formed iron hydroxide. Therefore, it can be concluded that the high value of the round-trip efficiency of the all-iron flow battery is achieved already starting from the first cycle and it is maintained during at least 20 (twenty) successive 6-hour cycles which confirms the achievement of the cyclic stability of the operation of the all-iron flow battery system.

[0085] The conducted researches confirm that use of the electrolytes according to the claimed invention allows to ensure a greater operation duration of the all-iron battery and a higher current density as compared to both non-stabilized and stabilized electrolytes known from the prior art.

Claims

CLAIMS1. A method for preparing a precursor for electrolytes of an all-iron flow battery, the method comprises steps of mixing an iron chloride crystal hydrate and a distilled water to obtain FeCl2solution, while simultaneously sparging an inert gas through the solution, separating the solution into two parts and further at least one step of reducing, in a first part of the solution, Fe ions to Fe ions, while simultaneously oxidizing, in a second part of the solution, Fe2+ions to form Fe3+ions by passing the parts of the solution through an electrochemical cell of the alliron battery during a charging process, while applying a current of 0.1-10 A per 1 cm of the electrochemical cell until values of a constant voltage of 1-1.4 V and a current of 0.5-5 A are achieved, and further transferring the reduced part of the solution to a container filled with inert gas or hydrogen.

2. The method according to claim 1, wherein the first part of the solution is 55-75% of a total volume of the FeCl2solution.

3. The method according to claim 1, wherein prior to the step of separating into two parts, a reducing agent is added to the FeCl2solution at a ratio of 1 : n, where n is a mass of the reducing agent and n = 0.05-0.005 of a mass of FeCl2, the obtained mixture is maintained for 1-24 hours at a temperature of 20-90°C in the inert gas or hydrogen atmosphere, while stirring continuously, and then the solution is filtered.

4. The method according to claim 1, wherein an iron powder and / or a hydrazine are / is used as the reducing agent.

5. The method according to claim 1, wherein argon and / or nitrogen are / is used as the inert gas.

6. A method for producing electrolytes for an all-iron flow battery, the method comprises steps of preparing a negative electrolyte by adding, to the reduced part of the solution obtained according to claim 1, an electrically conductive additive NH4C1 at a concentration of 0.5-4.0 M and a buffer additiveH3BO3at a concentration of 0.05-1 M, while adjusting pH to values in a range of 1.0-5.0, and possibly adding a distilled water and then adjusting the pH to values in a range of 2.0-4.0, and filtering, and all the steps are performed in an inert gas or hydrogen atmosphere, while stirring vigorously and heating to a temperature of 30- 80°C, and preparing a positive electrolyte by adding, to the reduced part of the solution, FeCl3-6H2O and the electrically conductive additive NH4CI at a concentration of 0.5-4.0 M, while adjusting pH to values in a range of 0.5- 1.0, and possibly adding a distilled water and then adjusting pH to values in a range of 0.0- 1.5, and filtering, and all the steps are performed under vigorous stirring and heating to a temperature of 30-80°C.

7. The method according to claim 6, wherein a volume of the negative electrolyte is greater than a volume of the positive electrolyte.

8. The method according to claim 6, wherein argon and / or nitrogen are / is used as the inert gas.

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